1. Field of the Invention
The present invention relates to an AF control apparatus which illuminates a subject with an auxiliary light, receives a reflection light from the subject to calculate distance information to the subject, and executes AF (automatic focusing) control, and an AF control method.
2. Description of the Background Art
In digital cameras such as digital video cameras and digital still cameras, light that has passed through an optical system having a variety of lens, for example, and formed an image (hereinafter, referred to as TTL light; TTL is an abbreviation for “through the lens.”) is detected by an image pickup sensor having an image pickup device such as CCD and CMOS, and subjected to various image processings after converted to a digital image signal (raw image data) by A/D conversion. The image data having subjected to these image processings is then displayed on an EVF (electronic view finder) or subjected to compression coding to be stored in a memory card, for example.
As an AF control system employed in digital cameras, a so-called “mountain climbing system” using TTL light is common which executes AF control in the manner as follows. An AF control mechanism mounted in a digital camera first extracts high frequency components Gh of the above-described digital image signal in a given AF area, and calculates a sum S0 of the same for storage. Next, the AF control mechanism calculates a sum S1 of high frequency components Gh′ after driving an optical system for moving the lens by a predetermined interval in one direction along the optical axis, and compares the sum S1 with the sum S0 previously calculated. When the sum S1 is larger than the sum S0 previously calculated, it is determined that the lens has moved close to a focusing position, and the lens is moved in the same direction. On the other hand, when the sum S1 is smaller than the sum S0, it is determined that the lens has moved away from the focusing position, and the lens is moved in the opposite direction. In this manner, the AF control mechanism drives the optical system until the sum of the high frequency components reaches near the maximum value and hence the image surface substantially coincides with the focusing surface. The driving method of optical system as described above is called a perturbation method. In other driving method which is called a full-range scanning method, the AF control mechanism moves the focal point of the lens from the minimum end to the infinite end across the full range while stopping the same little by little, and calculates a sum of the high frequency components at every stop position and stores the sum. Then, the AF control mechanism judges the lens position that corresponds to the value near the maximum value among the sums thus stored as a focusing position, and performs a control so as to move the lens to this focusing position. It is to be noted that in place of utilizing sums of high frequency components in the AF area, the maximum value of high frequency component in the same AF area may be utilized in some cases.
In general, the above-described mountain climbing system has a disadvantage that the time required for focusing (focusing time) is long while having an advantage of high AF accuracy.
Furthermore, in the case where a subject which is located at a short distance and contains less high frequency components and a subject which is located at a long distance and contains more high frequency components exist in the same AF area, the latter subject is more likely to become in focus. However, the main subject on which it is intended to focus is often located at a short distance, and in such a case, an out-of-focus background such that the background of the main subject is in focus will occur. Furthermore, in the case where these subjects are substantially the same in contrast, such a problem is posed that the position of focal point keeps transiting between the subject at a short distance and the subject at a long distance and hence the AF process will never end.